It has been recognized that many printing substances, such as ink and toner (as used in xerography and laser printing) is, in essence, an adhesive particularly suited to adhere to paper and other web material. Applicant's U.S. Pat. No. 5,213,560, which is expressly incorporated by reference herein, discloses a method and apparatus for utilizing printing toner, applied by a printing device, as a mechanism to seal a folded sheet to form a completed envelope or package. According to this disclosure, a sheet is provided with printed text and strategically-placed strips (of various printed patterns) of toner, typically along edges of the sheet. The toner strips are subsequently heated and the sheet is folded so that the toner strips overlap a folded section of the sheet. The toner is sealed under heat and pressure to create a completed sealed envelope or package. When the toner cools, the sealed envelope can be mailed like a conventional preformed envelope having contents therein.
By using toner as an adhesive, mailers can be produced from single sheets, reducing waste that normally results from the use of separate preformed envelopes. The single sheets can be derived from a continuous source of web such as a roll, expediting the mailer production process. The roll can be driven continuously through a printing device which lays down both text and adhesive toner strips in one step. Further perforations can be provided before or after printing that enhance the ease of opening the finished mailer. As disclosed in applicant's above-described patent, windows can also be formed in the sheets that include toner spots to facilitate the tacking of the window edge to an opposing surface of the mailer.
It has been contemplated that toner can provide an adhesive for binding a plurality of sheets together to form a book or pamphlet. In this instance, toner is applied in a strip to an edge of each sheet and the sheets are overlayed in groups while the edge is heated to cause the toner strips to fuse to adjacent sheet edges. Such a binding method is disclosed in U.S. Pat. No. 4,398,986 to Smith Jr., et al. This method involves the grouping of several sheets together in the presence of a heated platen that melts and the toner. A disadvantage of this method is that the platen often provides extreme heat to the outer sheets of the stack while not sufficiently heating the innermost sheets of the stack. When several groups of prebound stacks are, subsequently, grouped together, the uneven heating is more pronounced hence, the practical thickness of sheets of a bound stack utilizing this method is relatively small.
It is, therefore, an object of this invention to provide a method and apparatus for binding sheets utilizing printing toner that provides a uniform binding strength to all sheets in a stack regardless of stack thickness.